All currently operating internal combustion piston engines operating on the Otto cycle have their compression ratio and thus thermal efficiency limited by the fuel octane number. To prevent pre-ignition or detonation before top dead center the compression ratio is usually limited within the range from 8 to 11. Its compression ratio and thus thermal efficiency is further reduced when operated at less than full power because then a throttle valve is needed to maintain a near stoichiometric fuel-air mixture for efficient spark ignition.
All currently operating internal combustion piston engines, which operate on the Diesel cycle, have their thermal efficiency limited by near constant pressure heat addition during the combustion event which is quantified by the ratio of the cylinder volume at the end of combustion to the cylinder volume at the top dead center, called cut-off ratio (rc). This is determined by the time required for fuel injection and burning rate or cetane number. Compression ignition in the Diesel engine allows operation over a wide range of fuel-air ratios therefore a throttle valve is not required. Its thermal efficiency decreases with power level, as high power requires a larger cut-off ratio, which is contrary to the Otto cycle where the thermal efficiency increases with throttle opening and thus power level.
The Diesel engine cold starting problem is caused by increased fuel viscosity resulting in poor fuel injector atomization. Further the lower temperature of the air inside the cylinder reduces the fuel vaporization rate and thus ignitability.